Hot-dip galvanized steel sheet and manufacturing method thereof

ABSTRACT

A hot-dip galvanized steel sheet includes a steel sheet and a hot-dip galvanized layer arranged on the steel sheet, in which the Si content and the Al content by mass % of components of the steel sheet satisfy a relationship 0.5&lt;Si+Al&lt;1.0, and a metallographic structure of the steel sheet satisfies a relationship of {(n 2 ) 2/3 ×d 2 }/{(n 1 ) 2/3 ×d 1 }×ln(H 2 /H 1 )&lt;0.3 when the n 1  is the number of a MnS of a surface portion of the steel sheet, the d 1  μm is an average equivalent circle diameter of the MnS in the surface portion of the steel sheet, the H 1  GPa is a hardness of a martensite of the surface portion of the steel sheet, the n 2  is the number of the MnS of a center portion of the steel sheet, the d 2  μm is an average equivalent circle diameter of the MnS in the center portion of the steel sheet, and the H 2  GPa is the hardness of the martensite of the center portion of the steel sheet.

This is a Continuation of application Ser. No. 13/993,581 filed on Jun.12, 2013, which is the U.S. National Phase of PCT/JP2011/079045, filedDec. 15, 2011, which claims priority under 35 U.S.C. 119(a) to JapanesePatent Application No. 2010-281690, filed Dec. 17, 2010, the contents ofall of which are incorporated by reference, in their entirety, into thepresent application

TECHNICAL FIELD

An embodiment of the present invention relates to a hot-dip galvanizedhigh strength steel sheet having improved formability and amanufacturing method thereof. The hot-dip galvanized high strength steelsheet also includes a galvanneald high strength steel sheet.

Priority is claimed on Japanese Patent Application No. 2010-281690,filed Dec. 17, 2010, the content of which is incorporated herein byreference.

BACKGROUND ART

The strength increase (high tension) of a steel sheet used is one of themost effective methods to achieve both weight saving and a collisionsafety for a vehicle body. Recently, a regulation regarding thecollision safety represented by Euro-N-CAP has been stricter. In orderto correspond to the regulation, addition of a stiffened member or thelike is required, and thus, an increase in body weight is unavoidable.The increase in body weight results in a decrease in fuel efficiency.Accordingly, the increase in a utilization of an ultra-high strengthmaterial, in which a thickness is capable of being thinned while astrength of a part is maintained, has been more preferable. On the otherhand, in order to achieve the weight saving of the part as much aspossible, a shape of the part becomes complicated. Thereby, furtherimprovement of forming workability is required in the steel sheet.Particularly, in most cases, a high strength thin steel sheet is appliedto a portion, in which bending deformation is mainly performed, such asa side sill. Accordingly, it is important to estimate a holeexpansibility which is an index indicating a bendability or localductility as formability of the high strength thin steel sheet.Moreover, since corrosion resistance is also required in the member, ahot-dip galvanizing or a galvannealing is applied to the high strengthsteel sheet used.

However, in general, if the strength of the steel sheet is increased, itis known that the forming workability such as the bendability or thehole expansibility is deteriorated.

For example, in the related art, steel sheets having improved the holeexpansibility are suggested in Patent Documents 1 to 3.

Since the high strength steel sheet has much content of an alloyingelement and the alloying element is concentrated in a center portion ofa sheet thickness, the hole expansibility is deteriorated. However,there is no related art which discloses a hardness difference between asurface portion of the steel sheet and a center portion of the steelsheet. Moreover, since a MnS having a large size becomes a fractureorigin at the time of molding, it is assumed that a precipitation stateof the MnS influences formability. However, there is no related artwhich discloses the precipitation state of the MnS.

PRIOR ART DOCUMENT

[Patent Document]

-   -   [Patent Document 1] Japanese Unexamined Patent Application,        First Publication No. 2005-256141    -   [Patent Document 2] Japanese Unexamined Patent Application,        First Publication No. 2006-274317    -   [Patent Document 3] Japanese Unexamined Patent Application,        First Publication No. 2008-240123

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

An object of an embodiment of the present invention is to solve theproblems of the related art as described above, and to provide a hot-dipgalvanized steel sheet having high strength and improved formability anda manufacturing method thereof.

Means for Solving the Problems

The inventors eagerly reviewed a hot-dip galvanized high strength steelsheet having improved formability.

As a result, with respect to a steel sheet which is a DP (Dual Phase)steel having relatively low yield stress in high strength steel sheetsand becomes a substrate of a hot-dip galvanized steel sheet, bycontrolling a total amount of a Si and a Al, which are components of thesteel sheet, to a specific range and by controlling a hardnessdistribution of the steel sheet, the inventors found that a hot-dipgalvanized high strength steel sheet is capable of obtaining moreformability than the related art could be industrially manufactured.

In order to prevent problems such as a delayed fracture or secondaryworking embrittlement, it is preferable that the steel sheet be a DPsteel which substantially does not include a residual austenite exceptfor the residual austenite of approximately 5% by volume which isinevitably included.

Moreover, in the surface portion of the steel sheet and the centerportion of the steel sheet, it is important to control the hardness ofthe martensite or the precipitation state of the MnS in the steel sheet.

The aspect of the present invention may be applied to a hot-dipgalvanized high strength steel sheet having a tensile strength of 590MPa to 1500 MPa. However, remarkable effects are exerted on the hot-dipgalvanized high strength steel sheet having the tensile strength ofapproximately 980 MPa.

The gist of the present invention is as follows.

(1) According to an embodiment of the present invention, there isprovided a hot-dip galvanized steel sheet including: a steel sheet; anda hot-dip galvanized layer arranged on the steel sheet, wherein acomponent of the steel sheet includes, by mass %, C: 0.05% to 0.13%, Si:0.2% to 0.8%, Mn: 1.5% to 3.1%, P: 0.001% to 0.06%, S: 0.001% to 0.01%,N: 0.0005% to 0.01%, and Al: 0.1% to 0.7%, wherein the balanceconsisting of Fe and unavoidable impurities, wherein a Si content and anAl content by mass % satisfy the following Equation A, wherein ametallographic structure of the steel sheet includes a ferrite and amartensite, and wherein the metallographic structure satisfies thefollowing Equation B when the number of a MnS per 0.1 mm² on a surfaceportion of the steel sheet which is a region of ⅛ to 2/8 in a sheetthickness direction is n₁, an average equivalent circle diameter of theMnS on the surface portion of the steel sheet is d₁ μm, a hardness ofthe martensite of the surface portion of the steel sheet is H₁ GPa, thenumber of the MnS per 0.1 mm² on a center portion of the steel sheetwhich is a region of ⅜ to ⅝ in the sheet thickness direction is n₂, anaverage equivalent circle diameter of the MnS on the center portion ofthe steel sheet is d₂ μm, and the hardness of the martensite in thecenter portion of the steel sheet is H₂ GPa.0.5<Si+Al<1.0  (Equation A){(n ₂)^(2/3) ×d ₂}/{(n ₁)^(2/3) ×d ₁}×ln(H ₂ /H ₁)<0.3  (Equation B)

(2) In the hot-dip galvanized steel sheet according to (1), thecomponent of the steel sheet may further include by mass % at least oneof B: 0.0005% to 0.002%, Mo: 0.01% to 0.5%, Cr: 0.01% to 0.5%, V: 0.001%to 0.1%, Ti: 0.001% to 0.1%, Nb: 0.001% to 0.05%, Ca: 0.0005% to 0.005%,and Rare Earth Metal: 0.0005 to 0.005%.

(3) In the hot-dip galvanized steel sheet according to (2), the steelsheet may be a cold rolled steel sheet.

(4) In the hot-dip galvanized steel sheet according to (1), the steelsheet may be a cold rolled steel sheet.

(5) In a manufacturing method of the hot-dip galvanized steel sheetaccording to any one of (1) to (4), when a total number of stands in ahot finish rolling is n stage and r_(i)% is a rolling reduction of thei^(th) stand, the hot finish rolling may satisfy the following EquationC.(r ₁ +r ₂ +r ₃)/(r _(n-2) +r _(n-1) +r _(n))>1.6  (Equation C).

Advantage of the Invention

According to the hot-dip galvanized steel sheet related to the aspect ofthe present invention and the manufacturing method thereof, bycontrolling a total amount of the Si and the Al which are components ofthe steel sheet, to a specific range, and further by controlling thehardness of the martensite and the precipitation state of the MnS of thesteel sheet in the steel sheet which becomes a substrate of the hot-dipgalvanized steel sheet, a hot-dip galvanized steel sheet having the highstrength and improved formability and a manufacturing method thereof iscapable of being provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view showing a relationship between contents of the Al andthe Si in a steel sheet and steel sheet characteristics such asformability or a plating property.

FIG. 2 is a view showing a relationship between a value of the left sideof an Equation B, which represents a hardness of a martensite and aprecipitation state of a MnS in a surface portion of the steel sheet andin a center portion of the steel sheet, and a value of λ×TS whichestimates the strength and the formability of the steel sheet.

FIG. 3 is a view showing a relationship between a value of the left sideof an Equation C, which represents a control state of a rollingreduction at the time of starting and ending of a hot finish rolling,and the value of λ×TS which estimates the strength and the formabilityof the steel sheet.

FIG. 4 is a view showing a relationship between the value of the leftside of an Equation B, which represents the hardness of the martensiteand the precipitation state of the MnS in the surface portion of thesteel sheet and the center portion of the steel sheet, and the value ofthe left side of an Equation C which represents the control state of therolling reduction at the time of starting and ending of the hot finishrolling.

EMBODIMENT OF THE INVENTION

A hot-dip galvanized steel sheet according to an embodiment of thepresent invention includes a steel sheet which becomes a substrate and ahot-dip galvanized layer arranged on the steel sheet. Here, the hot-dipgalvanizing also includes a galvannealing.

First, a numerical limit range regarding the base elements of the steelsheet which becomes a substrate and the reasons for the limit will bedescribed. Here, the described % is mass %.

C: 0.05% to 0.13%

C (carbon) is an essential element in order to secure a strength andstabilize a martensite.

When the C content is less than 0.05%, the strength is not satisfied,and the martensite is not formed. Moreover, when the C content exceeds0.13%, a hardness of the martensite becomes too high, a hardnessdifference between a soft phase and the martensite becomes too large,and thus, a hole expansibility is decreased, and the weldability isdeteriorated.

Therefore, the C content is 0.05% to 0.13%, and is preferably 0.06% to0.1%.

Si: 0.2% to 0.8%

Si (silicon) is an element which is added in order to secure thestrength and a ductility.

When the Si content exceeds 0.8%, a phosphatability or a galvanizabilityis deteriorated. Accordingly, the Si content is set to 0.8% or less.Moreover, when the galvanizability is emphasized, the Si content ispreferably 0.6% or less. Furthermore, the hole expansibility is improveddue to multiple addition of Si and Al. However, when the Si content isless than 0.2%, the effect does not remarkably appear. Therefore, alower limit of the Si content is 0.2%.

Mn: 1.5% to 3.1%

Mn (manganese) is an element which delays a generation of a carbide andstabilizes a ferrite in addition to securing the strength.

When the Mn content is less than 1.5%, strength is not satisfied, aformation of the ferrite is insufficient, and thus, the ductility isdeteriorated. Moreover, when the Mn content exceeds 3.1%, thehardenability is increased more than necessary, and thus, the productquality is not stabilized. Furthermore, the ductility is also deficient.

Accordingly, the Mn content is 1.5% to 3.1%, and is preferably 1.8% to2.8%.

P: 0.001% to 0.06%

P (phosphorus) is added as an element which increases the strength ofthe steel sheet, if necessary. However, if the additional amount of P isincreased, an intergranular segregation occurs, and thus, localductility and the weldability is deteriorated.

Accordingly, an upper limit of the P content is 0.06%. Moreover, a lowerlimit of the P content is 0.001%, the reason is because costs areincreased on steel making process, if the P content is less than 0.001%.Preferably, the P content is 0.003% to 0.03%.

Al: 0.1% to 0.7%

Al (aluminum) is an element which promotes the generation of the ferriteand effectively acts on improvement of the ductility. In addition, Al isan element which does not deteriorate the phosphatability or thegalvanizability, even if the additional amount of Al is large.

In order to improve the hole expansibility by a multiple addition of Siand Al, the Al content is 0.1% or more. However, if the additionalamount of Al is increased, and causes the increase of inclusions such asan alumina, accordingly, the workability is deteriorated. Therefore, anupper limit of the Al content is 0.7%. Preferably, the upper limit is0.15% to 0.6%.

In addition to the above-described base elements, the steel sheet mayfurther include at least one of B, Mo, Cr, V, Ti, Nb, Ca, and Rare EarthMetal (REM) as selective elements. Hereinafter, the numerical limitranges of the selective elements and the reasons for the limit will bedescribed. Here, the described % is mass %.

B: 0.0005% to 0.002%

B (boron) may be added in order to secure the hardenability and increaseeffective Al due to a formation of BN. Moreover, the fraction of ferriteof DP steel is increased, and thus, a high elongation is capable ofbeing secured. However, the metallographic structure is a laminatedstructure, and thus, the local ductility may be decreased. Due toaddition of B, an aforementioned phenomenon is capable being prevented.

When the B content is less than 0.0005%, the effects are not exerted.When the B content exceeds 0.002%, the effects corresponding to theadded amount are not exhibited.

Mo: 0.01% to 0.5%

Cr: 0.01% to 0.5%

Mo (molybdenum) and Cr (chromium) may be added so as to secure thestrength and the hardenability.

When the Mo content and the Cr content are less than 0.01%, the effectsare not exerted. When the Mo content and the Cr content exceed 0.5%, aferrite generation is suppressed in the DP steel, and thus, theductility is deteriorated.

Moreover, the phosphatability or the galvanizability is deteriorated.

V: 0.001% to 0.1%

Ti: 0.001% to 0.1%

V (vanadium) and Ti (titanium) may be added so as to secure thestrength.

When the V content and the Ti content are less than 0.001%, the effectsare not exerted. When the V content and the Ti content exceed 0.1%, theweldability or the like is deteriorated.

Nb: 0.001% to 0.05%

Nb (niobium) may be added so as to secure the strength.

When the Nb content is less than 0.001%, the effects are not exerted.When the Nb content exceeds 0.05%, the effects are saturated.

Ca: 0.0005% to 0.005%

REM: 0.0005% to 0.005%

Ca (calcium) and REM may be added to suppress the generation ofinclusions and to improve the hole expansibility.

When the Ca content and the REM content are less than 0.0005%, theeffects are not exerted. When the Ca content and the REM content exceed0.005%, the effects are saturated.

In addition to the elements, the steel sheet includes unavoidableimpurities. Here, the unavoidable impurities show an auxiliary rawmaterial such as steel scrap or elements such as S, N, Mg, Pb, Sb, Sn,Cd, Ni, or Cu which is inevitably mixed in a plating process.

For example, even when Sn or the like is included within a range of0.01% or less, the effects of the present invention are not reduced.However, in order to sufficiently exert the effects of the presentinvention, it is preferable that the S content and the N content belimited as follows. Here, the described % is mass %.

S: 0.001% to 0.01%

S (Sulfur) is an element that unwelcomes to exist in the steel. Sgenerates MnS, and thus, the local ductility and the weldability aredeteriorated by it.

Accordingly, an upper limit of the S content is limited to 0.01%.Moreover, since costs in the steel making process are increased, if S isdecreased in an unnecessary manner. Thus, a lower limit of the S contentis 0.001%. Preferably, the S content is 0.002% to 0.005%.

N: 0.0005% to 0.01%

N (nitrogen) is an element which is inevitably included. However, if toomuch N is included, not only aging properties are deteriorated but alsoan amount of an AlN precipitation is increased, and thus, the effects ofAl addition are decreased.

Accordingly, an upper limit of the N content is limited to 0.01% orless. Moreover, if the N content is decreased in an unnecessary manner,since costs in the steel making process are increased, a lower limit ofthe N content is 0.0005% or more. Preferably, the N content is 0.001% to0.005%.

Next, the numerical limit range regarding the total content of Si and Alwill be described.

In order to increase the strength of the steel sheet, utilizations ofonly solid solution hardening or precipitation strengthening isinsufficient, generally, in many cases, transformation strengthening isalso used. Furthermore, since the controlling of a DP structure making,high elongation is obtained due to a soft ferrite phase, and thestrength is secured due to a hard martensite phase.

However, in a case of the DP steel, if a hardness difference between thesoft phase and the hard phase is increased, it is known that the holeexpansibility is deteriorated. In order to improve this, there is amethod which produces the decrease in hardness by tempering themartensite. However, the method is not sufficient enough. Particularly,in the DP steel which requires the tensile strength of 980 MPa or more,the strength may be deficient due to the tempering.

Thus, the inventors eagerly reviewed, and as a result, found that asteel sheet which has high strength and improved hole expansibility isobtained when the total amount of Si and Al by mass % satisfies thefollowing Equation A.0.5<Si+Al<1.0  (Equation A)

In evaluation of the strength and the formability, a value of λ×TS,which is the product of a hole expansion ratio: λ(%) and the tensilestrength: TS (MPa), is used. Generally, the value is approximately40,000% MPa. When the value of λ×TS is 60,000% MPa or more, itdetermined that the steel sheet has the high strength and improvedformability. FIG. 1 shows a relationship between the amounts of Al andSi by mass %, and steel sheet characteristics such as the formability orcoating performance. In FIG. 1, a steel sheet in which the value of λ×TSis 60,000% MPa or more is represented by “◯”, a steel sheet in which thevalue of λ×TS is less than 60,000% MPa is represented by “X”, and asteel sheet in which the galvanizability is deteriorated is representedby “Δ”. Furthermore, a range indicated by the Equation A is representedby an arrow “A”. As shown in FIG. 1, if the value of Si+Al by mass % is0.5% or less, the value of λ×TS is not sufficient, and if the value ofSi+Al is 1.0% or more, the galvanizability is deteriorated. In this way,the value of Si+Al by mass % is needed to be more than 0.5% and lessthan 1.0%. Preferably, the value of Si+Al by mass % is more than 0.6%and less than 1.0%.

Next, the metallographic structure of the steel sheet, which becomes asubstrate of the hot-dip galvanized steel sheet according to theembodiment of the present invention, will be described.

In the metallographic structure of the steel sheet, the ferrite and themartensite are mainly included. This is because the steel sheet hasimproved balance between the strength and the ductility. Here, theferrite includes a polygonal ferrite and a bainitic ferrite, and themartensite also includes a martensite obtained by performing temperingat a temperature of 600° C. or less in addition to a martensite obtainedby general quenching. Even when the steel sheet subjected to temperingat the temperature of 600° C. or less is used as the substrate of thehot-dip galvanized steel sheet, the effects of the present invention arenot changed.

The fraction of ferrite and the fraction of martensite are controlledaccording to the strength of the steel sheet. When TS is 500 MPa to 800MPa, it is preferable that the fraction of the ferrite be 50 area % to90 area % and the fraction of the martensite be 10 area % to 40 area %.When TS is 800 MPa to 1100 MPa, it is preferable that the fraction ofthe ferrite be 20 area % to 60 area % and the fraction of the martensitebe 30 area % to 60 area %. When TS exceeds 1100 MPa, it is preferablethat the fraction of the ferrite be 30 area % or less and the fractionof the martensite be 40 area % or more.

The steel sheet includes a bainite as the structure in addition to theferrite and the marteniste. It is preferable that the fraction ofbainite be 10 area % to 40 area %. Moreover, if an austenite remains inthe structure, secondary working embrittlement or delayed fracturecharacteristics are deteriorated. Accordingly, it is preferable thatresidual austenite be not substantially included in the steel sheetexcept for residual austenite of approximately 5 volume % whichinevitably exists.

In order to obtain high strength and sufficient hole expansibility(formability) in the steel sheet, the condition of the metallographicstructure is needed to satisfy the following Equation B.{(n ₂)^(2/3) ×d ₂}/{(n ₁)^(2/3) ×d ₁}×ln(H ₂ /H ₁)<0.3  (Equation B)

Here, the number of a MnS per 0.1 mm² on a surface portion of the steelsheet which is a region of ⅛ to 2/8 in a sheet thickness direction isn₁, an average equivalent circle diameter of the MnS is d₁ (m) and ahardness of the martensite of the surface portion of the steel sheet isH₁ (GPa). Similarly, the number of the MnS per 0.1 mm² on a centerportion of the steel sheet which is a region of ⅜ to ⅝ in the sheetthickness direction is n₂, the average equivalent circle diameter of theMnS is d₂ (μm), and the hardness of the martensite in the center portionof the steel sheet is H₂ (GPa).

The left side of the Equation B being less than 0.3 shows that thedifference of the numbers of the MnS, the difference of the averageequivalent circle diameters of the MnS, and the difference of martensitehardness in the surface portion of the steel sheet and the centerportion of the steel sheet are qualitatively small. Generally, thevalues of the number of the MnS, the average equivalent circle diameterof the MnS, and the martensite hardness in the center portion of thesteel sheet are larger than those in the surface portion of the steelsheet, and thus, the left side of the Equation B becomes 0.3 or more.

As shown in FIG. 2, there is a correlation between the value of the leftside of the Equation B and the value of λ×TS. When the value of the leftside of the Equation B is less than 0.3, the value of λ×TS becomes60,000% MPa or more. In this way, in order to obtain high strength andimproved formability in the steel sheet, the value of the left side ofthe Equation B is needed to be less than 0.3. Moreover, the lower limitof the Equation B which is assumed to be in general conditions is 0.01.

The hardness of the martensite and the precipitation state of the MnS inthe steel sheet are capable of being controlled by a manufacturingmethod described below. Furthermore, in order to more precisely controlthe hardness of the martensite and the precipitation state of the MnS inthe steel sheet, it is preferable that the steel sheet be a cold rolledsteel sheet in which cold rolling is also performed after hot rolling.

Moreover, if a value of EL×TS, which is the product of the elongation(EL) and TS, is 16,000% MPa or more, since the formability is furtherimproved, it is preferable that the value be 16,000% MPa or more.

As described above, by controlling the component and the metallographicstructure of the steel sheet in the steel sheet which becomes thesubstrate of the hot-dip galvanized steel sheet, the hot-dip galvanizedhigh strength steel sheet having high strength and improved formabilityare capable of being obtained.

Next, a manufacturing method of the hot-dip galvanized steel sheetaccording to the embodiment of the present invention will be described.

The manufacturing method may be performed by processes of a hot rolledsteel sheet, a cold rolled steel sheet, and a plating steel sheet whichare performed generally.

In a casting process, steel products are manufactured by casting ofmolten steel which satisfies the base elements, the selective elements,and the unavoidable impurities described above. The casting method isnot particularly limited, and a vacuum casting method, a continuouscasting method, or the like may be used.

In the hot rolling process, the hot rolling is performed by heating thesteel products. In order to prevent a decrease of the workability due toapply of a strain to ferrite grains excessively, a finish rolling in thehot rolling is preferably performed at temperature of Ar₃ (thetemperature in which ferrite transformation starts at the time ofcooling) or more. Moreover, since a recrystallized grain diameter afterannealing coarsens more than necessary at too high temperature, thefinish rolling in the hot rolling is preferably performed at atemperature of 940° C. or less.

At the time of the finish rolling in the hot rolling, when a rollingreduction at each stand satisfies a following Equation C, a high valueof λ×TS is obtained.(r ₁ +r ₂ +r ₃)/(r _(n-2) +r _(n-1) +r _(n))>1.6  (Equation C).Here, the number of a total stands of the hot finish rolling is n stageand r_(i)% is the rolling reduction of the i^(th) stand.

As shown in FIG. 3, there is a correlation between a value of the leftside of the Equation C and a value of λ×TS. When the value of the leftside of the Equation C exceeds 1.6, the value of λ×TS becomes 60,000%MPa or more. Accordingly, the value of the left side of the Equation Cpreferably exceeds 1.6. This is because it is assumed that thetemperature of the material to be processed is high at the time ofstarting the finish rolling, and if the rolling reduction is high atthis step, grains are uniform. On the other hand, since the temperatureof the material to be processed is low at the time of ending the finishrolling, if the rolling reduction is high at this step, load to thematerial to be processed is increased, and disorder in the shape occurs.Furthermore, at this step, since the inner portion of the material to beprocessed has uneven temperature distribution, dispersion in theprocessing is increased, and material characteristics are deteriorated.

In order to securely obtain high strength and improved formability, itis preferable that (r₁+r₂+r₃)/(r_(n-2)+r_(n-1)+r_(n))>1.9 be satisfied.Moreover, it is more preferable that(r₁+r₂+r₃)/(r_(n-2)+r_(n-1)+r_(n))>2.0 be satisfied. On the other hand,due to limitations of a plant capacity, an upper limit of the value of(r₁+r₂+r₃)/(r_(n-2)+r_(n-1)+r_(n)) becomes 3.0.

Moreover, as shown in FIG. 4, there is a correlation between the valueof the left side of the Equation B and the value of the left side of theEquation C. In FIG. 4, a steel sheet in which the value of λ×TS is60,000% MPa or more is represented by “◯” and a steel sheet in which thevalue of λ×TS is less than 60,000% MPa is represented by “X”. When bothof the Equation B and the Equation C satisfy respective conditions, thevalue of λ×TS becomes 60,000% MPa or more. That is, when steel productswhich satisfy the above-described components are used and the rollingconditions represented by the Equation C are satisfied, themetallographic structure of the steel sheet is satisfied, and as aresult, the value of λ×TS becomes 60,000% MPa or more.

Furthermore, if the value of EL×TS of the steel sheet is 16,000% MPa ormore, since the hot-dip galvanized steel sheet is capable of beingapplied to an automotive members or the like in which strict workabilityis required, it is more preferable that the value of EL×TS be 16,000%MPa or more.

As a winding temperature of the steel sheet after the hot rolling isincreased, recrystallization or grain growth is promoted, and theworkability may be improved. However, as the winding temperature isincreased, scales are generated, a pickling property is decreased. So, aferrite and a pearlite are generated in layers, and thus, C isnon-uniformly segregated. Accordingly, the winding temperature is set to650° C. or less. On the other hand, if the winding temperature is toolow, the steel sheet is hardened, and thus, a load at the time of coldrolling becomes high. Therefore, the winding temperature is set to 400°C. or more. Moreover, if necessary, the steel sheet after the hotrolling may be maintained at the winding temperature within a range of 1hour or more and 24 hours or less. The steel sheet is maintained duringthe time, and thus, the metallographic structure of the hot rollingsteel sheet is capable of being appropriately controlled.

If necessary, in a grinding process, in order to remove scales, surfacegrinding may be performed to the steel sheet after the hot rollingprocess. The grinding method is not particularly limited, and forexample, a wire brush roll, an abrasive belt, a shot blasting, or thelike may be used.

In a pickling process, the steel sheet after the hot rolling process orafter the grinding process is pickled. The pickling method is notparticularly limited, and an established pickling method which usessulfuric acid, nitric acid, or the like may be used.

In a cold rolling process, the steel sheet after the pickling process iscold-rolled. The cold rolling method is not particularly limited. In thecold rolling, since shape correction of the steel sheet is difficult ifthe rolling reduction is low, the lower limit of the rolling reductionis preferably 30%. Moreover, if the rolling is performed at the rollingreduction exceeding 70%, due to an occurrence of cracks in an edgeportion of the steel sheet and a deformation in the shape, the upperlimit of the rolling reduction is preferably 70%.

In the cold rolled steel sheet which is manufactured through the hotrolling process and the cold rolling process described above, thehardness of the martensite and the precipitation state of the MnS in thesteel sheet is more precisely controlled. Accordingly, the steel sheetis preferably used as the substrate of the hot-dip galvanized steelsheet.

In an annealing process, the steel sheet after the cold rolling processis annealed at the temperature of Ac1 (a temperature in which anaustenite starts to be generated at the time of heating) or more and Ac3(a temperature in which transformation from a ferrite to an austenite iscompleted at the time of heating) +100° C. or less. At the temperatureless than Ac1, the structure is nonuniform. On the other hand, at thetemperature exceeding Ac3+100° C., ferrite generation is suppressed dueto coarsening of the austenite, and elongation characteristics aredeteriorated. Moreover, from an economical aspect, the annealingtemperature is preferably 900° C. or less. Moreover, during theannealing process, in order to make layered structures disappear, thesteel sheet is needed to be maintained for more than 30 seconds.However, even when the steel sheet is maintained more than 30 minutes,the effects are saturated, and thus, the productivity is decreased.Accordingly, the maintaining duration is set to 30 seconds or more and30 minutes or less.

In a cooling process, the steel sheet, which is heated within thetemperature range in the annealing process, is cooled. A cooling endtemperature is set to 600° C. or less. If the cooling end temperatureexceeds 600° C., the austenite easily remains, problems about thesecondary workability and the delayed fracture may easily occur.Furthermore, if necessary, at the cooling end temperature, the steelsheet after the annealing process may be maintained within a range of 10seconds or more and 1000 seconds or less. According to the maintenancefor the time, the metallographic structure of the steel sheet after theannealing process is capable of being appropriately controlled.

Moreover, in order to improve the hole expansibility and thebrittleness, tempering treatment may be performed to the steel sheet attemperature of 600° C. or less after the cooling process, if necessary.Even when the tempering treatment is performed, effects of the presentinvention are not changed.

In a plating process, hot-dip galvanizing is performed to the steelsheet after the cooling process or the tempering treatment. A hot-dipgalvanizing method is not particularly limited. Moreover, if necessary,an alloying treatment is performed, and thus, a galvannealing may beused.

Example 1

Steel having the component composition shown in Table 1 was casted in avacuum melting furnace. In Table 1, underlined numerical values indicatevalues outside the range of the present invention. The steel productswere heated to 1200° C. and the hot rolling was performed. The finishrolling in the hot rolling was performed at 880° C. Moreover, in the hotfinish rolling, the rolling reduction was controlled at each stand.After the hot finish rolling ends, the rolling steel sheet was cooled to500° C. and was maintained for 1 hour at the temperature, and thewinding heat treatment of the hot rolling was performed. The surfacescales of the obtained hot rolled steel sheet were removed by grindingand pickling. Thereafter, the cold rolling was performed to the steelsheet. Annealing was performed to the steel sheet after the cold rollingfor 60 seconds at 800° C. by using a continuous annealing simulator.Thereafter, the steel sheet was cooled at a temperature range of 400° C.to 600° C. and maintained for 10 seconds to 600 seconds at thetemperature. Hot-dip galvanizing was performed to the steel sheet, andif necessary, some of the steel sheets were cooled down to a roomtemperature after being treated by the alloying treatment.

A tensile test and a hole expansion test were performed using thehot-dip galvanized steel sheet manufactured described above. When theproduct of the hole expansion ratio λ(%)×the tensile strength TS (MPa)was 60,000% MPa or more, it was determined that the steel sheet had highstrength and improved formability. The tensile test was performed by JISNo. 5 specimen. The hole expansion test is performed by pushing aconical punch having a tip angle of 60° into a punched hole which isprovided on the specimen and has an initial hole diameter d₀: 10 (mm),and by expanding the punching hole. Moreover, the hole diameter d (mm)was measured at the time when cracks generated at a circumference of thepunched hole penetrate in a sheet thickness direction of the specimen,and the hole expansion ratio λ was obtained by a following Equation D.λ=[(d−d ₀)/d ₀]×100(%)  (Equation D)

Here, d₀=10 mm.

The metallographic structure of the hot-dip galvanized steel sheetmanufactured as described above was observed by an optical microscope.An observed surface was a cutting section which was cut into a planeface along the sheet thickness direction so that a sheet width directionperpendicular to the rolling direction of the hot-dip galvanized steelsheet was the observed surface. The Ferrite was observed by NITALetching and the martensite was observed by Le Pera etching method. Aposition of ¼ of the thickness of the steel sheet, which was positionedat the steel sheet side from an interface which was shown on the cuttingsection and was between the steel sheet and the hot-dip galvanizedlayer, was observed, and area fractions of the ferrite and themartensite were obtained. After the surface of the hot-dip galvanizedsteel sheet was ground in a parallel manner to the depth which was ¼ ofthe sheet thickness of the steel sheet, a polished surface was measuredby an X-ray diffractometer, and thus, the volume faction of theaustenite was obtained.

A galvanizing property was estimated by performing hot-dip galvanizingto the rolling steel sheet, which was subjected to annealing under theannealing conditions similar to the above-described conditions, using ahot-dip galvanizing simulator, and by visually confirming an attachmentsituation of the plating. A case where the plated surface was 90 area %or more and the galvanizing was uniformly attached was represented by“Good”, and a case where the plated surface was more than 10 area % anddefects existed was represented by “Bad”. The results are shown in Table2.

An observation of the precipitation state of MnS was performed by usinga Field Emission-Scanning Electron Microscope (Fe-SEM). The observationwas performed at the surface portion of the steel sheet which was ⅛ to2/8 in the sheet thickness direction of the steel sheet from theinterface, which was shown on the cutting section and between the steelsheet and the hot-dip galvanized layer, to the steel sheet side, and atthe center portion of the steel sheet which was ⅜ to ⅝ in the sheetthickness direction of the steel sheet. A Magnification of theobservation was set to 1,000 times, and an area of 0.12 mm×0.09 mm=0.01mm² was set to one observation visual filed. A total of 10 visual fieldswere observed, and the number of the MnS was measured. Here, a total of10 visual fields for every area of 0.01 mm² were observed, the totalnumber was measured, and thus, the number of the MnS was represented bythe number per 0.1 mm². An equivalent circle diameter (μm) of the MnSwas calculated by an image analysis software in which the equivalentcircle diameters in the 10 visual fields were incorporated into theFe-SEM, the average value in the 10 visual field was obtained, and thus,the average value was set to the average equivalent circle diameter(μm).

The hardness of the martensite was measured using a nanoindenter. Grainsof the martensite, which existed on the surface portion of the steelsheet and the center portion of the steel sheet, were measured at total30 points with intervals of 100 μm, and the average value was obtained.The results were shown in Table 3. In Table 3, underlined numericalvalues indicate values outside the range of the present invention.

As shown in Tables 1 to 3, No. 1 to 27, which are Examples, are hot-dipgalvanized steel sheets which have improved the galvanizability, thehigh strength, and sufficient hole expansibility (formability).

On the other hand, Nos. 28 to 45, which are Comparative Examples, arehot-dip galvanized steel sheets outside the range of the presentinvention.

In Comparative Examples 28 and 29, since the C content is outside therange of the present invention, the value of λ×TS becomes less than60,000% MPa.

In Comparative Example 30, since the Si content is outside the range ofthe present invention, the value of Si+Al by mass % also is outside therange of the present invention, and thus, the value of λ×TS becomes lessthan 60,000% MPa, and the galvanizing property is also not good.

In Comparative Example 31, since the Si content and the Mn content areoutside the range of the present invention and the value of Si+Al bymass % also is outside the range of the present invention, the value ofλ×TS becomes less than 60,000% MPa, and galvanizing property is also notgood.

In Comparative Example 32, since the Mn content is outside the range ofthe present invention, the value of λ×TS becomes less than 60,000% MPa.

In Comparative Example 33, since the P content is outside the range ofthe present invention, the value of λ×TS becomes less than 60,000% MPa.

In Comparative Example 34, since the S content is outside the range ofthe present invention, the value of λ×TS becomes less than 60,000% MPa.

In Comparative Example 35, since the N content is outside the range ofthe present invention, the value of λ×TS becomes less than 60,000% MPa.

In Comparative Example 36, since the Al content is outside the range ofthe present invention, the value of λ×TS becomes less than 60,000% MPa.

In Comparative Examples 37 to 41, since the value of Si+Al by mass % isoutside the range of the present invention, the value of λ×TS becomesless than 60,000% MPa.

In Comparative Examples 42 to 45, since the Equation B and the EquationC are not satisfied, the value of λ×TS becomes less than 60,000% MPa.

TABLE 1 Chemical Component (mass %) C Si Mn P S N Al Cr Mo V Ti Nb Ca BREM Si + Al Example 1 0.051 0.250 1.65 0.005 0.008 0.0035 0.625 — — — —— — — — 0.875 2 0.052 0.202 2.02 0.023 0.006 0.0064 0.555 — — — — — — —— 0.757 3 0.055 0.288 2.50 0.008 0.009 0.0055 0.512 — 0.15 — — — — — —0.800 4 0.061 0.421 1.52 0.007 0.007 0.0035 0.444 — — — — — — — — 0.8655 0.052 0.256 1.55 0.008 0.008 0.0033 0.526 0.210 0.11 — — — — — — 0.7826 0.111 0.222 1.69 0.006 0.009 0.0087 0.623 — — — — — 0.004 — — 0.845 70.125 0.650 1.52 0.032 0.005 0.0042 0.250 — 0.15 — — — — — — 0.900 80.079 0.256 1.53 0.044 0.001 0.0040 0.666 0.320 0.05 — — — 0.003 — —0.922 9 0.095 0.475 1.62 0.008 0.002 0.0065 0.235 — — — — — — — — 0.71010 0.077 0.245 1.77 0.007 0.009 0.0022 0.321 — 0.25 — — — — — — 0.566 110.091 0.321 1.56 0.006 0.007 0.0015 0.222 — 0.11 — — — — — — 0.543 120.095 0.356 2.09 0.012 0.006 0.0035 0.565 — 0.21 — — — — — — 0.921 130.105 0.215 1.82 0.011 0.005 0.0022 0.623 0.390 — — — — — — — 0.838 140.101 0.235 2.68 0.009 0.008 0.0035 0.421 — 0.23 — — — — 0.0015 — 0.65615 0.128 0.625 1.92 0.023 0.007 0.0034 0.368 — — — — — — — — 0.993 160.069 0.568 2.99 0.005 0.001 0.0024 0.251 — 0.05 — — — — — — 0.819 170.125 0.515 1.66 0.011 0.003 0.0037 0.121 — 0.11 — — 0.01 0.002 0.0010 —0.636 18 0.111 0.458 2.03 0.016 0.004 0.0041 0.323 — — — — 0.03 — — —0.781 19 0.124 0.256 1.93 0.013 0.007 0.0034 0.135 — 0.12 — — — — —0.0020 0.391 20 0.115 0.689 2.95 0.018 0.003 0.0025 0.223 — 0.21 — 0.03— — — — 0.912 21 0.123 0.468 2.41 0.016 0.003 0.0064 0.356 — — — — — —0.0008 — 0.824 22 0.115 0.452 2.19 0.014 0.005 0.0007 0.238 — — — — — —— — 0.690 23 0.125 0.264 1.54 0.013 0.003 0.0087 0.333 0.50  0.11 — 0.05— — — — 0.597 24 0.126 0.521 2.35 0.022 0.007 0.0090 0.321 — — — — — —0.0015 — 0.842 25 0.128 0.777 2.66 0.050 0.008 0.0069 0.215 — 0.15 0.03— — — — — 0.992 26 0.129 0.352 2.85 0.041 0.005 0.0065 0.356 — 0.22 — —— — — — 0.708 27 0.126 0.450 3.00 0.038 0.003 0.0034 0.369 — 0.31 — —0.02 — — — 0.819 Comparative 28 0.040 0.235 1.52 0.007 0.008 0.00350.521 — — — — — — — — 0.756 Example 29 0.250 0.225 2.15 0.003 0.0060.0007 0.512 — — — — — — — — 0.737 30 0.125 1.523 2.35 0.007 0.0090.0035 0.356 — 0.15 — — — — 0.0006 — 1.879 31 0.116 1.498 1.30 0.0090.003 0.0032 0.621 0.280 0.32 — — — — — — 2.119 32 0.112 0.235 3.250.009 0.004 0.0034 0.678 — — — — — — — — 0.913 33 0.099 0.321 2.12 0.0750.003 0.0021 0.325 0.300 0.16 — — 0.01 — — — 0.646 34 0.062 0.455 2.500.002 0.020 0.0059 0.412 — — — — — — — — 0.867 35 0.055 0.356 1.55 0.0110.010 0.0210 0.253 — — — — 0.02 — — — 0.609 36 0.125 0.500 1.95 0.0180.004 0.0093 0.003 — 0.15 — — — — — — 0.503 37 0.126 0.210 2.65 0.0050.003 0.0022 1.923 — 0.22 — — — — — — 2.133 38 0.078 0.120 2.10 0.0080.003 0.0021 0.150 — — — — — — — — 0.270 39 0.128 0.920 2.35 0.008 0.0030.0021 1.150 — 0.35 — 0.01 — — — — 2.070 40 0.122 0.220 2.15 0.007 0.0030.0025 0.180 — — — — — — — — 0.400 41 0.115 0.650 2.22 0.008 0.0020.0033 0.520 — — — — — — — — 1.170 42 0.110 0.350 2.06 0.056 0.0030.0021 0.250 — 0.11 — — — 0.002 — — 0.600 43 0.078 0.520 1.55 0.0460.002 0.0030 0.110 — — — — — — — — 0.630 44 0.130 0.620 2.39 0.051 0.0060.0030 0.250 — 0.02 — — 0.01 — — — 0.870 45 0.121 0.220 2.25 0.005 0.0030.0030 0.680 0.210 0.03 — — — — 0.0010 — 0.900 *Underlined numericalvalues indicate values outside the range of the present invention.

TABLE 2 Evaluation Result Metallographic Structure Tensile Test and HoleExtpansiblity Test Residual Tensile Hole Ferrite Martensite AusteniteStrength Elongation Expansibility EL × TS λ × TS Fraction FractionFraction Galvanizing (MPa) (%) (%) (% MPa) (% MPa) (area %) (area %)(volume %) Property Example 1 577 33.2 105 19156 60585 68 22 2 Good 2576 32.5 125 18720 72000 68 23 3 Good 3 585 31.2 110 18252 64350 69 22 4Good 4 622 28.0 106 17416 65932 65 25 3 Good 5 777 22.3 95 17327 7381564 26 4 Good 6 798 23.2 86 18514 68628 59 33 3 Good 7 802 22.3 77 1788561754 58 31 5 Good 8 832 20.5 74 17056 61568 59 30 3 Good 9 845 20.0 7216900 60840 55 31 4 Good 10 855 21.0 75 17955 64125 52 31 3 Good 11 90120.0 71 18020 63971 52 36 4 Good 12 978 18.5 70 18093 68460 52 32 2 Good13 985 16.5 66 16253 65010 51 35 3 Good 14 990 16.5 62 16335 61380 50 344 Good 15 995 17.1 62 17015 61690 52 36 3 Good 16 1000 16.3 65 1630065000 55 38 3 Good 17 1002 16.4 66 16433 66132 52 41 3 Good 18 1005 16.368 16382 68340 51 36 4 Good 19 1008 16.4 61 16531 61488 48 38 4 Good 201012 19.2 65 19430 65780 44 41 3 Good 21 1022 18.5 60 18907 61320 42 425 Good 22 1023 17.2 67 17596 68541 37 44 4 Good 23 1045 16.5 62 1724364790 36 46 2 Good 24 1055 18.3 59 19307 62245 41 41 2 Good 25 1252 13.555 16902 68860 30 48 2 Good 26 1356 12.3 51 16679 69156 15 62 2 Good 271512 11.3 50 17086 75600 12 75 2 Good Comparative 28 405 32.1 65 1300126325 92 0 3 Good Example 29 1589 8.5 21 13507 33369 5 90 4 Good 30 98518.9 52 18617 51220 44 42 2 Bad 31 901 21.5 55 19372 49555 55 32 2 Bad32 1215 11.0 25 13365 30375 30 52 3 Good 33 804 21.3 45 17125 36180 5137 3 Good 34 602 24.6 74 14809 44548 68 21 4 Good 35 596 21.3 58 1269534568 69 21 4 Good 36 1352 10.5 33 14196 44616 21 68 4 Good 37 1367 10.332 14080 43744 22 69 3 Bad 38 877 16.5 45 14471 39465 25 65 3 Good 39985 17.2 45 16942 44325 72 11 4 Bad 40 1025 14.5 52 14863 53300 32 21 3Good 41 1052 17.6 41 18515 43132 56 12 5 Bad 42 1002 17.5 32 17535 3206442 30 3 Good 43 765 23.2 35 17748 26775 52 36 4 Good 44 885 10.2 42 902737170 48 38 3 Good 45 987 13.2 40 13028 39480 45 42 4 Good

TABLE 3 Evaluation Result Metallographic Structure Surface Portion ofSteel Sheet MnS Center Portion of Steel Sheet Average Average EquivalentEquivalent Number of Circle Martensite Number of Circle Martensite MnSDiameter Hardness MnS Diameter Hardness Value of n₁ d₁ H₁ n₂ d₂ H₂ LeftSide of (/0.1 mm²) (μm) (GPa) (/0.1 mm²) (μm) (GPa) Equation B Example 16 3 250 8 4 270 0.12 2 4 2 270 7 4 290 0.21 3 4 1 260 6 2 280 0.19 4 5 2280 7 3 310 0.19 5 7 2 290 8 2 300 0.04 6 7 1 320 7 1 330 0.03 7 6 3 3209 3 330 0.04 8 6 4 340 7 5 360 0.08 9 6 3 400 10 7 430 0.24 10 6 4 400 85 420 0.07 11 6 3 410 9 3 420 0.03 12 8 4 410 9 6 460 0.19 13 8 2 420 103 450 0.12 14 9 4 420 13 5 440 0.07 15 11 2 430 12 3 440 0.04 16 7 2 41010 5 430 0.15 17 6 1 440 8 3 460 0.16 18 8 2 430 10 2 440 0.03 19 9 4420 10 6 440 0.07 20 7 3 420 8 3 450 0.08 21 8 2 440 9 2 470 0.07 22 6 4440 10 4 480 0.12 23 10 2 440 12 2 460 0.05 24 10 5 450 14 6 470 0.07 2512 3 450 14 4 470 0.06 26 14 5 460 16 7 480 0.07 27 16 6 470 20 10 5000.12 Comparative 28 6 3 240 8 4 270 0.19 Example 29 12 7 510 14 8 5200.02 30 7 3 400 8 5 420 0.09 31 8 4 410 9 5 430 0.06 32 6 3 430 10 6 4600.19 33 4 3 380 6 6 400 0.13 34 6 4 300 8 6 320 0.12 35 7 4 300 9 5 3300.14 36 10 5 450 12 7 480 0.10 37 10 3 460 13 5 500 0.17 38 20 9 400 2413 420 0.08 39 10 7 420 12 9 440 0.07 40 12 6 420 15 8 480 0.21 41 11 6420 13 9 460 0.15 42 10 7 420 16 14 470 0.31 43 9 8 380 15 15 430 0.3344 8 8 390 10 15 450 0.31 45 8 7 410 14 15 460 0.36 ManufacturingCondition Rolling reduction in Hot Finish Rolling Value of r₁ r₂ r₃r_(n−2) r_(n−1) r_(n) Left Side of (%) (%) (%) (%) (%) (%) Equation CExample 1 51 40 38 30 29 12 1.82 2 50 40 38 30 25 20 1.71 3 51 38 30 2928 4 1.95 4 42 37 33 30 27 7 1.75 5 49 39 35 29 22 8 2.08 6 45 39 45 2523 4 2.48 7 49 39 35 23 21 7 2.41 8 49 38 36 30 21 8 2.08 9 43 40 36 2724 8 2.02 10 45 16 34 28 24 7 1.61 11 43 40 36 28 25 6 2.02 12 46 45 3325 23 4 2.38 13 48 40 35 23 20 8 2.41 14 47 41 35 28 23 8 2.08 15 51 4038 30 25 16 1.82 16 42 36 34 30 26 8 1.75 17 42 36 34 32 26 6 1.75 18 5040 39 30 27 14 1.82 19 51 38 30 29 27 5 1.95 20 47 41 35 29 23 7 2.08 2143 41 36 27 24 8 2.03 22 43 41 36 24 27 8 2.03 23 49 41 35 28 23 8 2.1224 46 45 33 25 23 4 2.38 25 47 41 35 29 23 7 2.08 26 51 38 30 29 28 41.95 27 43 41 36 24 27 8 2.03 Comparative 28 47 41 35 28 23 8 2.08Example 29 49 39 35 26 22 8 2.20 30 49 39 35 24 22 8 2.28 31 49 39 35 2622 8 2.20 32 43 41 36 24 27 8 2.03 33 51 38 40 30 29 12 1.82 34 51 38 3029 28 4 1.95 35 42 37 33 27 30 7 1.75 36 51 40 38 30 29 12 1.82 37 43 4136 27 24 8 2.03 38 49 39 35 26 20 10 2.20 39 49 39 35 24 22 8 2.28 40 4840 35 24 22 8 2.28 41 43 41 36 27 22 10 2.03 42 45 22 43 30 34 13 1.4343 38 30 29 43 33 7 1.17 44 45 43 22 34 30 13 1.43 45 48 36 18 40 35 71.24 *Underlined numerical values indicate values outside the range ofthe present invention.

INDUSTRIAL APPLICABILITY

According to the hot-dip galvanized steel sheet and the manufacturingmethod thereof of the aspects of the present invention, in the steelsheet which becomes a substrate of the hot-dip galvanized steel sheet,by controlling a total amount of Si and Al, which are components of thesteel sheet, to a specific range, and by controlling the hardness of themartensite and a precipitation state of the MnS of the steel sheet, ahot-dip galvanized steel sheet having high strength and improvedformability and a manufacturing method thereof is capable of beingprovided.

The invention claimed is:
 1. A hot-dip galvanized steel sheetcomprising: a steel sheet; and a hot-dip galvanized layer arranged onthe steel sheet, wherein a component of the steel sheet includes, bymass %, C: 0.05% to 0.13%, Si: 0.2% to 0.6%, Mn: 1.5% to 3.1%, P: 0.001%to 0.06%, S: 0.001% to 0.01%, N: 0.0005% to 0.01%, Al: 0.1% to 0.7%, anda balance including Fe and unavoidable impurities, wherein a content ofthe Si and a content of the Al by mass % satisfy a following Equation A,wherein the steel sheet has a tensile strength TS of 500 MPa or more andless than 800 MPa and a hole expansion ratio λ×TS is 60,000% MPa ormore, and a metallographic structure of the steel sheet includes aferrite and a martensite, wherein a fraction of the ferrite is 50 area %to 90 area % and a fraction of the martensite is 10 area % to 40 area %,wherein the metallographic structure satisfies a following Equation B,wherein n₁ represents a number of a MnS per 0.1 mm² on a surface portionof the steel sheet which is a region of ⅛ to 2/8 in a sheet thicknessdirection, d₁ represents, in μm, an average equivalent circle diameterof the MnS on the surface portion of the steel sheet, H₁ represents, inGPa, a hardness of the martensite of the surface portion of the steelsheet, n₂ represents the number of the MnS per 0.1 mm² on a centerportion of the steel sheet which is a region of ⅜ to ⅝ in the sheetthickness direction, d₂ represents, in μm, the average equivalent circlediameter of the MnS on the center portion of the steel sheet, and H₂represents, in GPa, the hardness of the martensite of the center portionof the steel sheet:0.5<Si+Al<1.0  (Equation A);{(n ₂)^(2/3) ×d ₂}/{(n ₁)^(2/3) ×d ₁}×ln(H ₂ /H ₁)<0.3  (Equation B). 2.The hot-dip galvanized steel sheet according to claim 1, wherein thecomponent of the steel sheet further includes, by mass %, at least oneof B: 0.0005% to 0.002%, Mo: 0.01% to 0.5%, Cr: 0.01% to 0.5%, V: 0.001%to 0.1%, Ti: 0.001% to 0.1%, Nb: 0.001% to 0.05%, Ca: 0.0005% to 0.005%,and Rare Earth Metal: 0.0005% to 0.005%.
 3. The hot-dip galvanized steelsheet according to claim 2, wherein the steel sheet is a cold rolledsteel sheet.
 4. The hot-dip galvanized steel sheet according to claim 1,wherein the steel sheet is a cold rolled steel sheet.
 5. A manufacturingmethod of the hot-dip galvanized steel sheet according to claim 1,wherein, when a total number of stands on a hot finish rolling is nstage, and r_(i)% is a rolling reduction at the i^(th) stand, the hotfinish rolling satisfies a following Equation C,(r ₁ +r ₂ +r ₃)/(r _(n-2) +r _(n-1) +r _(n))>1.6  (Equation C).
 6. Amanufacturing method of the hot-dip galvanized steel sheet according toclaim 2, wherein, when a total number of stands on a hot finish rollingis n stage, and r_(i)% is a rolling reduction at the i^(th) stand, thehot finish rolling satisfies a following Equation C,(r ₁ +r ₂ +r ₃)/(r _(n-2) +r _(n-1) +r _(n))>1.6  (Equation C).
 7. Amanufacturing method of the hot-dip galvanized steel sheet according toclaim 3, wherein, when a total number of stands on a hot finish rollingis n stage, and r_(i)% is a rolling reduction at the i^(th) stand, thehot finish rolling satisfies a following Equation C,(r ₁ +r ₂ +r ₃)/(r _(n-2) +r _(n-1) +r _(n))>1.6  (Equation C).
 8. Amanufacturing method of the hot-dip galvanized steel sheet according toclaim 4, wherein, when a total number of stands on a hot finish rollingis n stage, and r_(i) is a rolling reduction at the i^(th) stand, thehot finish rolling satisfies a following Equation C,(r ₁ +r ₂ +r ₃)/(r _(n-2) +r _(n-1) +r _(n))>1.6  (Equation C).